Pulse tube cooler having ¼ wavelength resonator tube instead of reservoir

ABSTRACT

An improved pulse tube cooler having a resonator tube connected in place of a compliance volume or reservoir. The resonator tube has a length substantially equal to an integer multiple of ¼ wavelength of an acoustic wave in the working gas within the resonator tube at its operating frequency, temperature and pressure. Preferably, the resonator tube is formed integrally with the inertance tube as a single, integral tube with a length approximately ½ of that wavelength. Also preferably, the integral tube is spaced outwardly from and coiled around the connection of the regenerator to the pulse tube at a cold region of the cooler and the turns of the coil are thermally bonded together to improve heat conduction through the coil.

(a) STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Goverment support under contact NAS5-02021awarded by NASA. The Goverment has certain rights in this invention.

(b) CROSS-REFERENCE TO RELATED APPLICATIONS

(Not Applicable)

(c) REFERENCE TO AN APPENDIX

(Not Applicable)

(d) BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to pulse tube cryocoolers and moreparticularly to a structure that can be substituted for the reservoirthat is used in common configurations and thereby reduce cost, workinggas volume, weight and cool down time.

2. Description of the Related Art

Traveling wave pulse tube coolers have been recognized as havingdesirable characteristics for cooling to cryogenic temperatures,particularly when multiple coolers are cascaded in stages. Theirdevelopment began with the study of the cooling effects resulting fromthe application of a pressure wave to one end of a tube that was closedat its opposite end. A regenerator was added to the tube and an exampleis illustrated in U.S. Pat. No. 3,237,421. The art recognized that thetime phasing between the pressure and the working gas mass flow velocityin the regenerator was critical to the heat pumping efficiency of thecooler. A dramatic improvement in performance resulted from the additionof an orifice, at the formerly closed end of the tube, with the orificeleading to a relatively large volume reservoir, also referred to as asurge volume, compliance volume or buffer. This orifice pulse tubecooler greatly improved the phasing in the regenerator therebyincreasing heat pumping efficiency. Numerous examples of the orificepulse tube cooler exist in the prior art of which U.S. Pat. No.5,794,450 is only one example.

The orifice and reservoir changed the acoustic impedance at the end ofthe tube and thereby changed the phase relationship between gas velocityand pressure. At the wall of a closed end of a tube, the boundarycondition velocity is always zero while the pressure oscillates andtherefore the closed end has a pressure anti-node and a velocity node.The closed end presents a nearly pure reactive impedance to the tube,with the pressure and velocity essentially 90° out of phase andreflecting energy. An orifice, however, when connected to a largevolume, that is sufficiently large that it does not undergo anysignificant pressure variation, allows gas to flow in oscillatingdirections through the orifice unaffected by a pressure change in thereservoir (because there is none) and allows pressure variations acrossthe orifice, if the orifice is not too large. Consequently, the combinedorifice and reservoir can be designed to present a resistive acousticimpedance to the tube. The resistive impedance has the characteristicthat the pressure and velocity of the gas at the orifice are in phase.The phasing change at the end of the tube resulting from substitution ofthe orifice and reservoir for the closed end wall resulted in a desiredchange in the phasing in the reservoir ultimately resulting in theimproved heat pumping efficiency.

Pulse tube coolers have also been configured with multiple cascadedstages as illustrated in U.S. Pat. No. 6,256,998 and U.S. Pub.2004/0000149.

The traveling wave pulse tube cooler was further improved bysubstitution of an inertance tube for the orifice. An example of thisconfiguration is illustrated in U.S. Pub. 2003/0226364. The inertancetube is a long narrow tube, typically a few meters long, that is open ateach end and can be wound in a coil. The inertance tube is connectedbetween, and inserts a reactive acoustic impedance between, thereservoir and the pulse tube. When connected in this manner to the pulsetube and cut to approximately ¼ wavelength of the acoustic wave, thiscombination presents a nearly resistive acoustic impedance to the end ofthe pulse tube. Using an inertance tube instead of an orifice, adesigner can, by varying the length of the inertance tube, vary theacoustic impedance, and therefore the pressure/velocity phasing, at theend of the pulse tube. This permits the designer more flexibility tofurther adjust and optimize the phasing in the regenerator and therebyfurther increase the heat pumping efficiency.

The reservoir, however, also has some undesirable characteristics. Thereservoir must enclose a large volume that is sufficiently large thatthe pressure of the gas within it does not vary appreciably throughoutan acoustic cycle. Furthermore, the reservoir must be sufficientlystrong that it will retain the working gas under the average pressure towhich the pulse tube cooler is charged. Therefore, the reservoir must bestructurally configured and have both its surface area and its thicknesssufficiently large to meet these requirements. As a consequence thereservoir has a large mass, has a large volume occupying considerablespace, is relatively heavy and is relatively expensive to manufacture.

Additionally, in multi-stage pulse tube cryocoolers, the upper stages(stages beyond the first stage) operate in their steady state at reducedtemperatures. In some implementations, the reservoir and inertance tubefor an upper stage operates at the temperature of its warm region or“end” which is at the temperature of the cold region or “end” of thepreceding stage. Therefore, under transient conditions when thecryocooler is cooling down to its operating temperature, the pulse tubecooler stages must cool down the reservoir as well as other components.The relatively large mass of the reservoir, and its consequent high heatstorage capacity, causes a substantial time delay until the cryocoolerreaches operating temperature.

It is therefore an object and feature of the invention to substitute forthe reservoir of a pulse tube cooler, a structure having a greatlyreduced mass and volume that is also considerably less expensive andeasily made from a readily available, common product, and can be moreeasily contained within the outer vacuum vessel in which cryocoolers areordinarily housed.

(e) BRIEF SUMMARY OF THE INVENTION

The reservoir of a pulse tube cooler is replaced by a resonator tubethat has a length substantially equal to ¼ wavelength of a standing wavein the working gas, or an odd, integer multiple thereof, at theoperating frequency, temperature and pressure of the resonator tube.Preferably, the resonator tube is formed integrally with the inertancetube as a single, integral tube serving the functions of both.

(f) BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art pulse tube cooler.

FIG. 2 is a schematic diagram of an embodiment of the invention.

FIG. 3 is a schematic diagram in partial vertical section of a preferredembodiment of the invention.

In describing the preferred embodiment of the invention which isillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific term so selected and it is to be understoodthat each specific term includes all technical equivalents which operatein a similar manner to accomplish a similar purpose. For example, theword connected or terms similar thereto are used. They are not limitedto direct connection, but include connection through other elementswhere such connection is recognized as being equivalent by those skilledin the art. In addition, devices are illustrated which are of a typethat perform well known operations. Those skilled in the art willrecognize that there are many, and in the future may be additional,alternative devices which are recognized as equivalent because theyprovide the same operations.

(g) DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 diagrammatically show pulse tube coolers in a U-tubeconfiguration, although the invention is applicable to linear and otherconfigurations. Those Figs. each show a single stage, but, as known inthe art, pulse tube coolers can have multiple stages cascaded with theeach stage accepting heat from its immediately subsequent higher stage,or if it is the highest stage from the object being cooled, andrejecting heat to its immediately preceding lower stage, or to theambient atmosphere, if it is the lowest stage. Therefore, the coolers ofFIGS. 1 and 2 also represent individual stages of a cryocooler havingmultiple, cascaded stages.

The pulse tube cooler of FIG. 1 is constructed in accordance with theprior art. A pressure wave generator, having a selected operatingfrequency such as 30 Hz or 60 Hz, is connected through a heat rejectingheat exchanger 12, a regenerator 14 and a heat accepting heat exchanger16 to one end of a pulse tube 18. The connection from the regenerator 14to the pulse tube 18 is through a turning manifold 20 that contains theheat exchanger 16. The opposite end of the pulse tube 18 is connected toa first end of an inertance tube 22 which, as known in the art, isordinarily constructed so that it is approximately ¼ wavelength long.However, as also known in the art, the inertance tube 22 typicallydeparts in length from precisely ¼ wavelength. The reason for thisdeparture is that it is undesirable to have a velocity node at the pulsetube end of the inertance tube because there would be no gas motion atsuch a node so the cooler would not work properly. The opposite end ofthe inertance tube 22 is connected to a compliance reservoir 24. Asknown in the art, there may be other heat exchangers and all of theseconnections are both mechanical connections and fluid communicationconnections. The cooler is charged with and contains a working gas, suchas helium, and has a selected operating temperature and operates at aselected mean pressure. The wavelength of acoustic waves in the workinggas is determined at the operating temperature and is affected to a muchlesser extent by pressure.

The embodiment of FIG. 2 differs from the cooler of FIG. 1 by thesubstitution of a substantially ¼ wavelength resonator tube 26 for thereservoir 24. The resonator tube 26 can be a separate structureconnected in fluid communication to the inertance tube 28 of FIG. 2. Itcan also have a different passage cross sectional area and shape.However, preferably, the resonator tube 26 is formed integrally with theinertance tube 26 so that together they form a single, integral tube.

Replacing the reservoir with the resonator tube of the invention hasseveral advantages. There is a large industry that makes tubing so it isa relatively fungible product that is readily and inexpensivelyavailable. There is no need to design and fabricate a reservoir tooperate at the required pressures and temperatures. The resonator tube26 encloses a considerably smaller volume and has considerably less massthan a reservoir and therefore not only has less weight and takes upless space, but also there is less mass to be cooled down to operatingtemperatures on start up when the pulse tube cooler is an upper stage ofmultiple cascaded stages. Therefore, cool down time is reduced. Becausethis also greatly reduces the total gas volume in the cooler, lessworking gas flows through the pulse tube, manifold and regeneratorduring cool down.

Additionally, because the appropriate tubing is conveniently available,and the resonator tube can be formed integrally as an extension of theinertance tube, all that is necessary is to cut a piece of tubing to alength that is substantially ¼λ longer than the designed inertance tubelength, sealing and closing one end and attaching the opposite end tothe pulse tube in the conventional manner. This provides essentially thesame pressure/velocity boundary conditions as desired and found in theprior art when the reservoir is used with the inertance tube.

The term “tube”, when applied to the ¼ wave resonator tube of theinvention, has a meaning ordinarily implied by the term “tube”. It is anelongated body enclosing a hollow interior passage that can contain afluid. Although most commonly cylindrical, it can have other polygonalcross sectional shapes, such as oval, square, triangular or rectangular.Its length is considerably greater than the lateral dimensions. Theimportant feature of the resonator tube used with the invention is thatit function to support a close approximation of an acoustic standingwave inside with a pressure-node and velocity anti-node at the endconnecting to the inertance tube and pressure anti-node and velocitynode at the opposite, far, closed end. The resonator tubecross-sectional area is not important to wave propagation but of courseits length should be an odd, integer multiple of a ¼ wavelength of astanding wave in the working gas within the resonator tube at theoperating frequency, temperature and pressure of the resonator tube sothat it supports the close approximation of a ¼ wavelength acousticstanding wave. It is desirable to minimize the size and weight of theresonator tube, the volume of working gas it contains and to have anegligible flow resistance. Excessive flow resistance reduces the coolerperformance. Excessive weight and tube diameter add weight to the coolerand make winding the tube in a coil difficult. Therefore, the resonatortube cross sectional area is chosen as an engineering tradeoff orcompromise by choosing a cross sectional area that avoids the excessiveflow resistance resulting from too small a cross sectional area and theexcessive size, weight and working gas volume resulting from too great across sectional area. We have, for example, used a 4 mm diameter tubeand find that it barely affects cooler performance and is small enoughto wind into a coil and not add excessive weight. Since the resonatortube is a substitute for a heavier reservoir, a net weight reduction isusually accomplished.

FIG. 3 shows a cascaded, two stage pulse tube cooler having a firststage cold head 31 and a second stage cold head 32. The first stage hasa pulse tube 34, turning manifold 36 and regenerator 38. The secondstage regenerator 40, having heat exchangers at its opposite ends, isconnected through a turning manifold 42 to the pulse tube 44. The secondstage 32 also has an integral tube 46 coiled around and spaced outwardlyfrom the turning manifold 42 of the second stage 32. The turningmanifold 42 in the illustrated embodiment is the second stage connectionof the regenerator to the pulse tube forming the cold region of thesecond stage cold head. An open end 48 of the coiled tube 46 isconnected to the pulse tube 44 and the opposite end 50 of the coiledtube 46 is closed. The coiled tube 46 has a total length approximately ½wavelength of acoustic waves. Specifically, the length of the tube 46 isthe sum of the ¼ wavelength long resonator tube segment of the coiledtube 46 that is located proximally from the pulse tube 44 and begins atthe closed end 50, added to the desired length of an inertance tubedesigned in accordance with the principle known in the art.

Advantageously, the turns of the tubular coil 46 are soldered or brazedtogether so they are held in place mechanically and are bonded togetheralong a continuous thermally conductive path. The coil is similarlybonded to an annular plate 52 that is mounted in thermal conduction tothe turning manifold 36 of the first stage. This mechanically retainsthe coil relatively rigid but more importantly provides a thermallyconductive path from the entire coil 46 to the cold region of the firststage 31. This thermally conductive path facilitates the conduction ofheat from the coil 46 during cool down of the pulse tube cooler.

There are, of course, many alternative ways to coil the tube around thecold head. The turns of the coiled tube can, for example, be woundaround or within a cylindrical inner or outer sleeve and can bethermally and mechanically connected to the sleeve.

While certain preferred embodiments of the present invention have beendisclosed in detail, it is to be understood that various modificationsmay be adopted without departing from the spirit of the invention orscope of the following claims.

1. An improved pulse tube cooler including a pressure wave generator,having a selected operating frequency, and connected through aregenerator to one end of a pulse tube, the opposite end of the pulsetube connected to a first end of an inertance tube, the cooler having aselected operating temperature and containing a working gas foroperating at a selected mean pressure, wherein the improvementcomprises: a resonator tube having a first end connected to theopposite, second end of the inertance tube and having an opposite,second end that is sealingly closed, the resonator tube having a lengthsubstantially equal to an odd, integer multiple of a ¼ wavelength of astanding wave in the working gas within the resonator tube at theoperating frequency, temperature and pressure of the resonator tube. 2.A pulse tube cooler in accordance with claim 1, wherein the integermultiplier is
 1. 3. A pulse tube cooler in accordance with claim 2wherein the resonator tube is formed integrally with the inertance tubeas a single, integral tube.
 4. A pulse tube cooler in accordance withclaim 3 wherein the length of the integral tube is substantially ½ ofsaid wavelength.
 5. A pulse tube cooler in accordance with claim 3wherein the integral tube is spaced outwardly from and coiled around theconnection of the regenerator to the pulse tube at a cold region of acooler that is at least a second stage of a multi-stage cooler.
 6. Apulse tube cooler in accordance with claim 5 wherein the coil has turnsthat are thermally bonded together to improve heat conduction throughthe coil.
 7. A pulse tube cooler in accordance with claim 2 wherein theinertance tube and the resonator tube are spaced outwardly from andcoiled around the connection of the regenerator to the pulse tube at acold region of a cooler that is at least a second stage of a multi-stagecooler.
 8. A pulse tube cooler in accordance with claim 7 wherein thecoil has turns that are thermally bonded together to improve heatconduction through the coil.